Can I use UV project modifier for a specific face?
The simplest way to get a clean separation in texturing between the faces is to set up two materials, one for the single face, and one for the rest of the mesh. Then you can set up a UV map for each material and they won't conflict with each other due to the face assignment of the materials.

In my example, I'm actually UV projecting textures for both materials even though you don't need to do that. Just set up the UV projection for the top face in order to achieve the result you want.



STEP ONE

Create 2 UV Maps (you can unwrap the whole mesh into both maps, as we are differentiating between the textures via face material assignment)



STEP TWO

Create 2 Materials Similar to the screenshot




STEP THREE

Apply the materials to the desired faces in Edit mode.



STEP FOUR

Set up the UV Projection modifier like this (you can just do 1 projection even though I've done 2). You'll also need a separate camera for projection (you can use the main camera but it's generally more convenient to use a separate camera)



STEP FIVE

Now you should have a different material and a texture that is UV projection on your mesh surfaces.